Devices and methods for controlling solenoid-valve-regulated fuel-metering systems are well known in the automotive technology.
German Patent Application No. 40 04 110 discloses a method and device for controlling a diesel engine using a solenoid-valve-regulated fuel pump. The fuel pump described therein includes a pump piston, driven by a camshaft, which pressurizes the fuel and delivers it to the individual cylinders. The beginning and end of fuel delivery are actuated by means of at least one solenoid valve. A control unit calculates trigger instants for the solenoid valve in accordance with markings configured on a shaft.
A problem with known systems arises by virtue of the fact that the fuel-metering control unit emits trigger signals based upon time quantities. The exact beginning of fuel injection must take place when the engine camshaft assumes a particular angular position. Injection ends when the camshaft has rotated through a specific angular displacement from its angular position at the beginning of injection. For this reason, time quantities must be converted into angular displacements and vice versa using the rotational speed of the camshaft as the conversion factor. The accuracy of this calculation depends directly on the accuracy of measuring the rotational speed.
In the prior art, the instantaneous rotational speed at the sampling instant immediately prior to calculation is used to convert between time quantities and angular displacements.
A method and device for so controlling a solenoid-valve-regulated fuel pump are described in German Patent Application No. 40 04 107. Given the desired starting point of injection and the desired duration of fuel delivery, the electronic control system, disclosed therein, determines the trigger instant and the shut-off instant for one or more solenoid valves. That system also accounts for the switching time of practical solenoid valves. The switching time of a solenoid valve is that time which elapses between the application of a triggering signal and the completed reaction of the solenoid valve. In the system described in the aforementioned patent, the switching times of the solenoid valves are handled as time quantities. Due to the fact that the switching time of the solenoid valves follows from a corresponding reduction in the residual times, the influence of the switching times is not adequately compensated for, causing inaccurate fuel-metering.
The method and device of the present invention overcome the inaccuracies of fuel-metering in known systems.